<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin:0;
            padding:0;
        }
        .provide{
            height:500px;
            width:500px;
            background-color: red;
            margin:100px;
            overflow: hidden;
            position: relative;
        }
        .parent{
            height:300px;
            width:300px;
            background-color: blue;
            margin:100px;
            overflow: hidden;
        }
        .box{
            height:100px;
            width:100px;
            background-color: aqua;
            /* 父(祖先)相子绝 */
            position: absolute;
            left:20px;
            top:20px;
        }
    </style>
</head>
<body>
    <div class="provide">
        <div class="parent">
            <div class="box"></div>
        </div>
    </div>
</body>
</html>